402 



USE OF THE GALVANOMETER 



until the abscissa of its intersection with the magnetic curve is equal 

 to the abscissa of the intersection of the curve of the current to be 

 measured, in its normal position, with the same curve. The sine of 

 the latter abscissa, divided by the sine of the former, expresses the 

 ratio of the current to be measured to that assumed as unity. 



Apart from geometric construction, this rule may be thus expressed. 

 To measure the force of a current which is greater or less than the 

 assumed unit, observe first the deflection produced by it when the 

 coils lie in the magnetic meridian. Then turn the coils, in the former 

 case backward, in the latter forward, until the angle between the 

 needle and the coils is equal to this angle of deflection. The sine of 

 this angle of deflection, divided by the sine of the angle of deflection 

 produced after the rotation of the coil, is the ratio of the current to 

 the assumed unit. 



It will be seen from this that if we have a table like the one already 

 given, containing, for a certain current assumed as unity, all the 

 values of n, or the angle made by the needle and the coil of wire, 

 from 0° to 80°,' and the corresponding values of to, or the angle of 

 the magnetic meridian with the coils, a second table may be con- 

 structed from this which will give the ratio of the deflection of the 

 needle to the current producing it, in terms of the same unit, for the 

 case where the coils lie in the magnetic meridian. 



As examples, I give two such tables constructed for my galva- 

 nometer. The current for the first was furnished by a small thermo- 

 pile, made of two pairs of German silver and copper wires twisted 

 together and heated at their alternate connexions in a sand bath 

 over a spirit lamp. During the eighteen measurements made, which 

 required not more than half an hour, the current was as good as 

 constant. These measurements were only undertaken to illustrate 

 the process, and I therefore sought neither after great accuracy nor 

 completeness. The values of n could easily have been determined 

 for every degree. 



The ratio between the deflections and the forces of the currents, 



